I absolutely love this shirt! I was amazed at the comfort and warmth and temperature regulation--I am never too hot or too cold, and I'm wearing it under a t-shirt at work in a fairly wide range of temperatures (from 45 degrees outside, with a light jacket over, to 70 degrees in a temp-controlled lab). I've slept in it too, and found I really do sleep better in wool, never getting either cold or sweaty despite being at that time of life. This was my "test" piece from Simply Merino and it is so terrific I've already ordered another as well as a long-sleeved shirt for me and a baby nightshirt for my granddaughter-to-be! I have a personal preference for more of a tank-shape than the slightly racerback style, but that's not a big enough deal to keep me from giving it 5 stars.